1. Field of the Invention
The present invention relates to an image forming technique for forming an image on a print medium.
2. Description of the Related Art
An ink-jet printing apparatus using a printhead having a plurality of ink discharge ports is known as one example of an apparatus that uses a printhead having a plurality of printing elements.
In an apparatus of this kind, there are instances where dots formed by ink undergo a variation in size and formation position owing to a fluctuation in the amount of ink discharged and the discharge direction thereof, and this can produce density unevenness in the printed image. In particular, in a serial-type printing apparatus that performs printing by causing a printhead to scan in a direction different from that in which the plurality of printing elements are arrayed (e.g., in a direction perpendicular to the element array direction), density unevenness ascribable to the above-mentioned fluctuation appears in the printed image as unevenness in the form of horizontal streaks. These streaks can be visually conspicuous and invite a decline in the quality of the printed image. Further, in order to correct for such density unevenness, a method proposed in cases where use is made of an ink-jet printhead having a plurality of discharge ports is to form one line of image data, which has undergone halftone processing (binarization processing, etc.), by ink discharged from a plurality of different discharge ports. This is achieved by performing paper feed of less than the width of the printhead and complementing the image data of one line with multiple scans or passes. This method generally is referred to as “multi-pass printing”. Multi-pass printing includes a method that uses a mask pattern and a method in which input image data that is to undergo multivalued printing is subjected to density partitioning in conformity with each pass and print data is generated for each pass of partitioned density. In all of these methods, input image data to be printed is printed upon being divided into a plurality of scans or passes.
An advantage of such methods is that since input image data to be printed is printed upon being divided into a plurality of scans or passes, there is less of a decline in image quality ascribable to horizontal streak-like unevenness caused by a variation in the characteristics of the printhead. However, a disadvantage is that printing time is lengthened correspondingly. For this reason, when a print medium is scanned with an ink-jet head, the general practice is to use bidirectional printing, in which printing is performed by back-and-forth scanning.
At present, pass division using a mask pattern generally is employed as the pass division method when such multi-pass printing is carried out. According to this method, initially the input image data is converted to print images of the respective ink colors, after which print data that relies upon a printhead is generated for every ink color. Then, when printing is actually performed, the printing of each pass is carried out upon taking the logical AND between the print data and the mask pattern. That is, the print data is generated in correspondence with each ink color based solely upon the input image data with no consideration being given to the printing method, such as the bidirectional printing method or multi-pass printing method.
Next, reference will be had to FIGS. 9A and 9B to describe how ink droplets that have been discharged from the ink-jet printhead of an ink-jet printer impact upon a print medium and are absorbed into and fixed to the print medium. FIG. 9A is a sectional view illustrating the state of absorption and fixation when ink droplets are discharged as adjacent pixels at a comparatively long time interval, and FIG. 9B is a sectional view illustrating the state of absorption and fixation when ink droplets are discharged as adjacent pixels at a comparatively short time interval. The diagram shown at the bottom of FIG. 9A is a top view illustrating the state of absorption and fixation of the ink droplets on the print medium. The diagram shown at the bottom of FIG. 9B is a top view illustrating the state of absorption and fixation of the ink droplets on the print medium.
In FIG. 9A, an ink droplet that has been discharged from the ink-jet printhead heads toward the print medium from above. When the discharged ink droplet impacts upon the print medium, the ink droplet is absorbed into the print medium. Next, after a comparatively long time interval, in a state in which the initial ink droplet has dried and become sufficiently fixed, an ink droplet is discharged as an adjacent pixel. Since the ink droplet that impacted first dries and is sufficiently fixed, the ink droplet that impacts subsequently partially creeps under the ink droplet that impacted and became fixed first. If the print medium is observed from above, it will be seen that the two ink droplets each impacted upon the print medium in the manner indicated at the bottom of FIG. 9A.
In FIG. 9B, an ink droplet that has been discharged from the ink-jet printhead heads toward the print medium from above in a manner similar to that shown in FIG. 9A. When the discharged ink droplet impacts upon the print medium, the ink droplet is absorbed into the print medium. Next, after a comparatively short time interval, in a state in which the initial ink droplet has not dried and become sufficiently fixed, an ink droplet is discharged as an adjacent pixel. When an ink droplet impacts as an adjacent pixel in a state in which the initial ink droplet has not dried and become sufficiently fixed, the two ink droplets are pulled together and drawn to the center by surface tension and merge into a single droplet. This resulting droplet is absorbed into the print medium, dries and becomes fixed. As a result, if the print medium is viewed from above, it is seen that only the central portion is dense, as shown at the bottom of FIG. 9B. Consequently, despite the fact that ink droplets were discharged twice, the resultant density on the print medium in FIG. 9B is less in comparison with the case where the ink droplet was discharged as the adjacent pixel after the initial ink droplet dried and became fixed sufficiently as in FIG. 9A. Thus, regardless of the fact that two ink droplets are discharged from the ink-jet printhead to form adjacent pixels, a difference develops in the density on the print medium owing to the ink-discharge time intervals.
In an effort to deal with this phenomenon in which a difference develops in printing density owing to the ink-discharge time intervals, a technique has been proposed in which, when the time interval becomes longer than usual owing to a recovery operation, etc., during the course of printing, subsequent printing is changed to have a dot size smaller than that of print data already generated [see the specification of Japanese Patent Laid-Open No. 2003-341022 (Document 1)].
Next, a procedure for performing two-pass printing on a print medium 200 will be described with reference to FIG. 10. FIG. 10 is a diagram illustrating the positional relationship between the print medium 200 and an ink-jet head 220.
The ink-jet head 220 has a nozzle portion 220-21 for performing printing of a first pass of two-pass printing, and a nozzle portion 220-22 for performing a second pass of the two-pass printing. A position 220-2 on the ink-jet head 220 is a relative position with respect to the print medium 200 when the second pass of printing is performed by the ink-jet head 220 following the first pass. The print medium 200 has an initial area 200-21 in which two-pass printing is performed on the print medium 200, and a second area 200-22 in which two-pass printing is performed on the print medium 200.
In an actual ink-jet printer, there is a mechanical mechanism for bidirectionally scanning an ink-jet head that employs a carriage (not shown). The ink-jet head is mounted on the carriage and is scanned back and forth above the print medium. In conformity with this bidirectional scanning of the ink-jet head mounted on the carriage, the print medium is transported by a transport mechanism (not shown). In actuality, the print medium is transported in the sub-scan direction that crosses the back-and-forth scanning direction (main-scan direction) of the ink-jet head mounted on the carriage. In order to simplify the description, however, in the next scan the ink-jet head 220 is scanned relative to the print medium 200 at the position 220-2.
In order to shorten printing time while using multi-pass printing, it is contemplated to achieve this by scanning the ink-jet printhead bidirectionally, as mentioned above. In the initial area 200-21 on the print medium 200, forward printing in performed from left to right by the nozzle portion 220-21 for performing the first pass of printing by the ink-jet head 220. Next, the print medium is transported and the ink-jet head 220 performs printing in the backward direction at position 220-2. At this time the second pass of printing is performed in the initial area 200-21 of the print medium 200 by the nozzle portion 220-22 for performing the second pass of printing by the ink-jet head 220. At the same time, the first pass of printing is performed in the second area 200-22 of the print medium 200 by the nozzle portion 220-21 for performing the first pass of printing by the ink-jet head 220.
When two-pass printing is performed, bidirectional printing by two scans of the ink-jet head 220 is performed in an area (e.g., 200-21) of the print medium 200, as described above, to thereby form an image. FIG. 11 illustrates this in the form of time.
FIG. 11 is a diagram illustrating the relationship between X coordinates and printing time in two-pass printing. The horizontal axis is a plot of the X coordinates of the print medium 200, and the vertical axis is a plot of printing time. With serial printing in which printing is performed by causing the ink-jet head 220 to scan above the print medium 200, an image is printed by discharging ink sequentially in the course of scanning the head forward or backward. Accordingly, the position scanned changes with the passage of time. That is, printing is performed on the right side of the print medium 200 at a time different from that at which printing is performed on the left side. A curve 230-1 in the graph of FIG. 11 is one representing X coordinates and printing times at these X coordinates when printing is performed by forward scanning. A curve 230-2 in the graph of FIG. 11 is one representing X coordinates and printing times at these X coordinates when printing is performed by backward scanning following printing by forward scanning according to curve 230-1.
As illustrated in FIG. 11, the time difference at identical X coordinates between the first pass of printing (curve 230-1) and the second pass of printing (curve 230-2) differs depending upon the X coordinates owing to bidirectional printing. Time difference (tl) on the left side of the print medium is longer than time difference (tr) on the right side. Further, as already shown in FIGS. 9A and 9B, the aforementioned phenomenon occurs in which density decreases when the printing time interval of adjacent pixels is short and increases when the printing time interval of adjacent pixels is long. This means that in a case where an image has been printed at uniform density, the densities on the right and left sides of the print medium will differ from each other. In this case, the density on the left side of the print medium will be high and that on the right side will be low. Although not illustrated, the first pass of printing in the area 200-22 of the print medium is in the backward direction and the second pass of printing in this area is in the forward direction, and therefore the size relationship in terms of the time differences between the right and left sides of the print medium reverses. The time differences on the left and right sides depend upon the scanning speed of the ink-jet head and the width of the print medium. In particular, the larger the width of the print medium, the greater the printing time differences and the more conspicuous the difference in density.
When printing is performed by scanning a print medium with an ink-jet head, if printing is carried out only at the time of forward scanning, the printing time interval when multi-pass printing is performed will be fixed and equal to the time required for the round trip regardless of the location on the print medium.
However, if printing is performed both forward and backward when a print medium is scanned with an ink-jet head, the time interval at which printing is performed by the ink-jet head differs depending upon the position on the print medium. That is, the printing time interval is short near where the ink-jet head changes direction between the forward direction and the backward direction and is long on the side opposite. This means that following printing with an initial droplet of ink, the printing with the next ink droplet will take place in a state in which the absorption and fixation of the initial ink droplet on the print medium has been completed or a state in which this absorption and fixation of the initial ink droplet on the print medium has not been completed.
With an ink-jet printer that forms an image by discharging ink onto a print medium, the density of color development differs depending upon the state of absorption and fixation of the printed ink. That is, even if printing is performed by multi-pass printing in which bidirectional printing is carried out with the same amount of ink, the final density that results from subsequent printing will vary depending upon the state of the ink printed first. As a result, the density and tint on the left side of the print medium will be different from that on the right side owing to bidirectional printing using the ink-jet head. Moreover, since the density difference between the left and right sides reverses whenever a round trip is made, the result is conspicuous band-type unevenness having a fixed width. In particular, such color unevenness becomes very noticeable in the case of a printer for printing on a print medium having a large width. FIG. 12 is a diagram illustrating such band-type unevenness.
Further, the printing time interval diverges greatly when recovery processing is executed during printing (recovery processing is processing, such as preliminary ink discharge or wiping, executed in order to maintain the head ink-discharge characteristic) and when printing is suspended unexpectedly (e.g., suspension due to the printer cover being opened by the user or suspension due to interruption in transfer of print data). As a consequence, color unevenness occurs when printing is resumed.
With the technique disclosed in Document 1, it is proposed that, when printing is performed following the occurrence of a time interval that is longer than usual owing to a recovery operation, etc., during printing, printing be carried out upon changing ink droplet size of print data from large dot to small dot without changing the print data already generated. As a result, when the time interval diverges greatly, lowering the printing density in the next scan is effective in eliminating the difference in printing density before and after.
However, although it is possible to suppress an increase in density in a case where the time interval has diverged greatly owing to recovery of the ink-jet head, it is not possible to suppress band-like unevenness, which is ascribable to a difference in printing density produced between both sides of the print medium, in a case where printing is performed in each of forward and backward scans. Furthermore, this difference in printing density reverses with each forward and backward scan and results in band-like unevenness that is very conspicuous.